Periodic domain inversion structure can be formed in a ferroelectric and non-linear optical crystal such as lithium niobate or lithium tantalite so as to generate second harmonic wave corresponding with the period. An optical waveguide can be easily formed in such crystals as lithium niobate or lithium tantalate, so that a device for generating second harmonic wave of high efficiency and small size can be provided. It is possible to generate light of a wavelength of ultraviolet, visible and infrared regions only with an excitation laser oscillating fundamental wave. The device can be applied in a wide variety of applications such as medical use, opt-chemical use and various kinds of optical measurement.
It is necessary to form deep polarization inversion structure in a ferroelectric single crystal, for attaining a high conversion efficiency in a second harmonic wave generation device. According to the method described in Japanese Patent publication No. H11-72809A, the surface of a substrate of a ferroelectric single crystal is inclined with respect to the polarization axis of the crystal by 3° and a comb electrode and rod-shaped electrode are formed on the surface of the substrate. Several low resistance portions are formed between the forward end of each electrode piece of the comb electrode and the rod-shaped electrode. Direct current voltage is then applied on the comb and rod-shaped electrodes, so that polarization inversion parts are formed corresponding with the electrode pieces of the comb electrode as well as the low resistance portions, respectively (FIG. 28).